I've had a post a few weeks back about lens choices and decided to look at the second hand market before I buy a new lens. My previous choices was between the Tamron 70-300 and Sigma 70-300. I want a zoom lens to shoot wildlife and birds in particular.

I've found a Sigma APO 170-500mm F5-6.3 DG that's still in a really excellent condition for a very good price.
I've read a view user comments on the web and there are a lot of different opinions on the image quality at higher zoom ranges. Everybody however seems to agree that the lens is quite heavy. Now my questions are:

1) From the people on this forum that have used this lens, what is your experience on the image quality?

2) How heavy is this lens?

3) Is this an auto focus (AF) lens? Will I be able to use AF after F5.6 with my 450d?

According to the above link: 1,345g/47.4 oz
Remember this is a 500mm and it's all relative - Check the weight of an EF-500 prime and this lens seems quite light (and small !!!)

Quote:

3) Is this an auto focus (AF) lens? Will I be able to use AF after F5.6 with my 450d?

With most lens of this type the lens fools the camera into registering f/5.6 (which is under-reporting ~1/3 of a stop) and the camera AF would still work... The IQ on this lens is no better than the Tamron 70-300 or Sigma 70-300 that you were looking at previoulsy. Plus their AF is also quite slow which is a waste really to spend any $ on... + some may need to be re-chipped because of their age!